The present invention relates to a luminescent device. Semiconductor lasers for use as a light source for optical communication systems or as a light source for information processing devices such as digital audio disks and video disks are shown in an article entitled "Technical Innovation in Semiconductor Laser" by Oka et al, pp. 25-29 of the publication "Semiconductor World", May, 1982.
The present applicant has already developed a communication laser module (semiconductor laser device) of the type shown, for example, in an article entitled "Semiconductor Lasers for Optical Communication Systems" by Hirao et al in "Hitachi Review" Vol. 1, 33 (1984) No. 4, pp. 193-198.
This semiconductor laser device is composed of a semiconductor laser element, an optical fiber which is fixed such that its end confronts the end surface (light emitting end surface) of the resonator of the semiconductor laser element, a light-receiving element for detecting the optical output of the laser beam emitted from the laser element (laser tip), and a hermetic package hermetically accommodating these elements and the optical fiber.
This semiconductor laser device functions satisfactorily as a semiconductor laser device for optical communication. However, the present inventors have found, through a study from the technical point of view, that this semiconductor laser device involves the following problems. It often happens that the portion of the optical fiber within the package is broken in a heat cycle test which is conducted after the assembly of device by subjecting the device to a repeated heat cycle of -45.degree. C. to 80.degree. C. for the purpose of confirming reliability.
According to the result of the study made by the present inventors, the breaking of the optical fiber is attributable to the following reasons.
The optical fiber mentioned before is composed of:
(1) A core disposed at the center of the optical fiber and made of quartz and a clad layer (referred to also as "core line", hereinunder); PA0 (2) Silicone resin coating the core and the clad layer; and PA0 (3) A fiber jacket made of, for example, nylon and covering the silicone resin.
In the process for producing this optical fiber, the core and the clad layer made of quartz is coated with the resin which is soft. During this coating step, strain is caused in the core and the clad due to differences in the thermal expansion coefficients and hardness, so that a microscopic undulation is imparted to the central axis of the optical fiber.
When a semiconductor laser device employing this optical fiber is subjected to the heat cycle test, the central axis of the optical fiber tends to resume its original straight state so that the core wire projects from the fiber jacket so as to cause a deflection of the central axis within the package. It is assumed that the optical fiber has been broken when this deflection has increased to exceed a predetermined limit.
The study made by the present inventors has proved also that, in some cases, the fiber may be broken by as few as 10 heat cycles in the aforementioned temperature test.
The present invention is based upon the result of this study.